Magnetic memory cells are memory cells that store information in the orientation of the magnetization of a ferromagnetic region. These magnetic memory cells are nonvolatile and can hold stored information for long periods of time. Magnetic memory cells that use a magnetic state to alter the electrical resistance of the materials near the ferromagnetic region are collectively known as magnetoresistive (MR) memory cells. An array of magnetic memory cells is often called magnetic RAM or MRAM (magnetic random access memory). MRAM arrays include an array of magnetic memory cells positioned at the intersections of wordlines and bitlines. Each cell includes a magnetically changeable or free region and a proximate reference region arranged into a magnetic tunnel junction (“MTJ”) device. The principle underlying storage of data in such cells is the ability to change the relative orientation of the magnetization of the free and reference regions by changing the direction of magnetization along the easy axis (“EA”) of the free region, and the ability to thereafter read this relative orientation difference. More particularly, MRAM cells are written to by reversing the free region magnetization using applied bi-directional electrical, and resultant magnetic, stimuli via its respective bitline and wordline, and are later read by measuring the resultant tunneling resistance between the bitline and wordline. The tunneling resistance assumes one of two values depending on the relative orientation of the magnetization of the free region with respect to the reference region. If the free region is modeled as a simple elemental magnet having a direction of magnetization which is free to rotate but with a strong preference for aligning in either direction along its easy axis (+EA or −EA), and if the reference region is, for example, a similar elemental magnet but having a direction of magnetization fixed in the +EA direction, then two states (and therefore the two possible tunneling resistance values) are defined for the cell: aligned (+EA/+EA) and anti-aligned (−EA/+EA). These two states may be used to represent a logical 1 or 0 for typical binary processing applications.
Coercivity refers to the magnetic field strength required to alter the orientation of the magnetization in a magnetic memory cell. In an MRAM device, the bits should all have approximately the same coercivity so that they can all be altered using a magnetic field of approximately the same strength. In addition, it is important that a magnetic memory be designed such that when the bits are scaled to smaller dimensions they do not suffer from thermally activated switching errors due to a decrease in their volume. The traditional approach uses shape anisotropy to create this needed coercivity. Unfortunately, this approach suffers from several problems. First, it is difficult to pattern all of the junctions with exactly the same shape. This variance in the shapes of the junctions leads to a corresponding spread in the coercivities of the bits. Furthermore, large aspect ratios are required to create this shaped-induced anisotropy and the large aspect ratios of the shaped anisotropic bits decreases the density of an MRAM constructed out of the bits. Finally, as the bits are scaled down in area, the thickness of the bits must also decrease in order to maintain the required coercivity. This scaling down of the thickness of the bits results in a decrease in bit volume which increases the likelihood of thermally activated switching errors. Therefore, what is needed is an improved magnetic memory bit that has a substantially uniform coercivity and can be scaled down without introducing thermally activated switching errors.